The present invention relates to a drive device for a double thread chain-stitch buttonhole sewing machine.
A known one of such drive devices comprises a standard electric motor having a rotor with two shaft ends. A high-speed drive acts on a first, so-called worm shaft. It has a jaw clutch. There is a sewing drive which acts on a second upper shaft. It also has a friction clutch. A mechanical disconnect device positions the sewing needle in its upper position after the end of the sewing process. A mechanical switching device alternately couples the sewing drive or the high-speed drive. The known drive device requires considerable expenditure for mechanical parts, particularly those parts which must be brought continuously into and out of engagement, as they are subject to considerable wear. Furthermore, disturbing switch noises occur in the known drive device. Also, positioning of the sewing needle in its upper position can practically never be achieved if the electric motor is disconnected during the sewing process, for instance, upon disturbances in the sewing.